As is known in the field of brushless DC motor controllers, rather complex electronic circuits are generally required to rotate the rotor by energizing and de-energizing the armature coils, or windings associated with the stator of the motor in sequence and with a particular time relationship (a process referred to generally as commutation). In conventional three-phase brushless DC motors, two windings are driven by being selectively connected to a positive or negative supply voltage via a respective pair of electronically controlled switches and a third winding is unenergized, or floating. As the rotor rotates relative to the stator, a voltage is induced in the motor windings, which is referred to as back electromotive force (i.e., back EMF).
Various techniques are employed for sensing the position of the rotor for purposes of commutation. In one such technique, the back EMF associated with the unenergized motor winding is measured to provide an indication of the rotor position. The back EMF of the unenergized winding is often sensed with the use of low pass filters and/or integrators. However, these circuits tend to be complex, sensitive to component values and relatively slow.
It is often necessary to provide a mechanism for regulating the current through the energized motor windings in order to regulate motor speed or torque. One way of regulating motor current is by pulse-width-modulating (PWM), or chopping, the drive signals which control the electronically controlled switches associated with the driven windings. In accordance with one such chopping scheme, either the driven switch connected to the negative voltage is always the chopped switch or the driven switch connected to the positive voltage is always the chopped switch.
In accordance with another chopping scheme, the switches of each switch pair associated with the driven windings are alternately chopped. This arrangement advantageously maintains the average voltage at the centertap terminal of the motor at one-half the supply voltage level. While these techniques are relatively efficient, both tend to be rather susceptible to noise. In order to reduce the resulting noise coupled to other circuits, the motor drive signals are often slew-rate limited and/or low-pass filtered using inductors, capacitors and/or resistors which adds to the cost and complexity of the circuit.